3GPP LTE (Long Term Evolution), usually referred to just as “LTE”, is a cellular radio access communication technology for packet switched services. A cell is defined by a set of resources for supporting terminals with communication. A radio base station (RBS) provides the physical entities for the cell resources. FIG. 1a illustrates a cellular system 100, for simplicity only one cell, 130, one RBS, 110, and one terminal 120 is shown, while in practice a cellular system comprises a plurality of RBS each supporting one or more cells, and there typically are a number of terminals in each cell. The presence of a radio channel 140 between the terminal 120 and the RBS 110 is also indicated in FIG. 1a. A cell is often described as serving terminals within a physical area, albeit, the service areas of different cells may fully or partly coincide with each other. In LTE the terminal is named UE (User Equipment) and the RBS eNodeB. In the further description the terminal is named UE, while the general acronym RBS is maintained.
In radio communication, and especially in cellular communication, noise and interference from non-desired signals are added to the received power of a desired signal. The desired signal may be detected when the received power of it exceeds that of noise and interference with a minimum level. Most transmissions from UEs are scheduled and therefore the RBS knows many details on when and how to best receive it. However, some UL (UpLink) radio channels are non-scheduled and the RBS has no expections when the UEs may transmit on the channel. Typically these types of channels are used by the UEs to transmit an access signal to request access to transmission resources. The uncertainty in timing, if any reception will be made at all, in combination with no error correction coding being available makes it difficult for the RBS to detect the access signals. The RACH (Random Access CHannel) is one example on these types of channels, and the problems will be more elaborated with the RACH as example, albeit the further problem description is also relevant for other channels that carry access signals.
The RACH is a non-scheduled channel used by a UE to communicate in the UL with the RBS. The RBS is typically not aware of if/when a UE will need to access the network, in particular for initial access, when the UE makes a first contact with the network, and when it has UL data to transmit after a period of non-transmission. The UE will transmit an access signal, here named random access preamble, as it wants to access the network.
The power control of the RACH in radio systems, i.e. the ability to control that the received power level for the RACH preamble is sufficiently high has always been a crucial part of radio network control. It is essential that also UEs on the cell edge can access the network. In case the threshold in the RBS for detecting an access signal is set too high, the cell range shrinks.
With the RACH type of channel, it is always a trade-off between missed detections and false alarms, which is illustrated in FIG. 1b. A missed detection on the RACH means that the RBS does not detect a random access preamble transmitted by a UE. If the UE has the capacity to increase the power it retransmits the random access preamble at higher power until a response is received. In such a way, access may eventually be achieved but at the expense of delay. A false alarm on the RACH means that the RBS incorrectly detects a random access preamble when none has been transmitted, e.g. due to a high noise and possibly interference peak. There is in other words a trade-off between service coverage and access delay on the one hand and false alarm load and resource consumption on the other.
In order to separate random access preambles which have actually been transmitted by real UEs from those falsely detected from noise and interference, a threshold parameter is needed. Typically, the threshold should be set such that a desired false alarm ratio is achieved.
It is normally a tedious work to designate the values of such a threshold parameter, since the noise and interference changes continuously over time and area and with that the required signal to interference plus noise ratio (SINR). A too high threshold as compared to the actually required SINR implies the number of access attempts made before the UEs succeed in getting access to the network will increase, and this in turn generate further interference. Hence, traffic can not be handled in the swift pace that would be desired. A too low value, on the other hand, implies a sensitive receiver that wastes its limited resources to serve noise rather than serving true traffic. Again, traffic can not be handled in the swift pace that would be desired.
Even though generally the RBS has no advance-knowledge of if, or when a UE will need to access the network there are some situations where the use of the random access channel can be foreseen. For situations when the RBS knows in advance when UEs will use the random access channel, e.g. at incoming handover, the RBS can assign dedicated random access preambles for explicit use by such individual UE. The random access procedure that applies with an assigned random access preamble is here referred to as contention free random access (CFRA), also called non-contention based random access, while contention based random access (CBRA) is used for the former where the UE randomly selects one preamble from a set of preambles allocated for CBRA.
The inevitable result of a system that uses both of these random access schemes, e.g. one that supports both public CBRA and designated CFRA, is that the random access preambles must be pre-partitioned in two groups: one group broadly announced to the UEs as being available for random selection in case a UE on its own initiative accesses the network and a second group consisting of those used to temporarily assign individual preambles as the CFRA users occur, see FIG. 5. The second group need typically not be broadly announced.
Problems with Existing Solutions on the RACH
There is an ever-changing trade-off between false alarm and missed detection. The existing methods to determine the best value of a threshold parameter are very much methods that involve trial and error.
The problems with existing solutions are:                1. Lack of Observability; A missed detection can not be observed since by definition it leaves no trace and there is no evidence it did occur. A false detection can not be observed in that way it can not easily be distinguished from a detection of a true access whenever such occurs on a channel that does not allow the completion of the random access procedure.        2. Coarse and uncertain estimations; There is no secure method to select as threshold value one equilibrium matching that specific SINR of detection which minimizes false detections as well as missed detections. Rather the methods are typically based on trial and error. Often a value that implies a substantial amount of false detections must be selected to achieve, if not optimal, so at least acceptable service coverage.        3. Direct impacts on live traffic; As can be seen in FIG. 1b, increasing the threshold used to detect real accesses will immediately increase the amount of missed detections. Missed detections imply longer delays and increased call setup times. Decreasing the threshold will directly increase the amount of false alarm detections.        4. Tedious and ever-changing; The noise and interference changes over time and area. It is a hard if not an impossible quest to find the thresholds of equilibrium that maximizes served traffic in different parts of the radio access network.        